Microsoft Security Essentials is another good free one to look at.
http://www.microsoft.com/security_essentials/
Just loaded it up to see how it goes. My FireFox is also freezing but I'm not sure that AV has anything to do with it. It could be flash, JAVA, my router, FireFox, and extension to FF or 2 dozen other things. Could also be my ISP but not likely as throughput seems OK for long downloads, etc.
Interesting that my ping (on two different PC's) has dropped from 33-35 to 19-21 since installing MS Security vs Avast 4.8. Only pause was the keyboard seemed to freeze when MS was doing a drive scan (letters appeared about 2 seconds after I typed them).
Just put it on my daughters desktop. This desktop has a wireless adapter. Ping went from 28 to 10ms and speed went from just under 10Mbps to 10.49Mbps. Impressed so far (and that was while scanning the HD).
I'm trying something other than Avast to see if my issues go away and the fact that Avast released a corrupted database this week that caused thousands of false positives and nearly caused me to wipe my drive. Many had to because it corrupted their OS drives so badly from deleting files that they had no choice.